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Q:

What are the restrictions when overriding a method ? 

Answer:



Q:

When do you override hashcode and equals() ?

Answer

HashCode method return int value. So the Hash value is the int value returned by the hash function .


If you want to do equality check or want to use your object as key in HashMap, we must override hashcode and equals() method.

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Q:

Which of these class have only one field 'TYPE' ?

A) Run time B) Process
C) Void D) System
 
Answer & Explanation Answer: C) Void

Explanation:

The Void class has one field, TYPE, which holds a reference to the Class object for the type void.

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Q:

 What is the string contained in s after following lines of code?

StringBuffer s = new StringBuffer(“Hello”); s.deleteCharAt(0);  ?

A) llo B) Hllo
C) ello D) H
 
Answer & Explanation Answer: C) ello

Explanation:

deleteCharAt() method deletes the character at the specified index location and returns the resulting StringBuffer object.

So after deleting the character at 0 ie 'H', the string returns 'ello' as the output.

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Q:

Which of the following are incorrect form of StringBuffer class constructor  ?

A) StringBuffer(int size , String str) B) StringBuffer(int size)
C) StringBuffer(String str) D) StringBuffer()
 
Answer & Explanation Answer: A) StringBuffer(int size , String str)

Explanation:
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Q:

Does not overriding hashcode() method has any performance implication ?

Answer

A poor Hashcode() function will result in frequent collision in HashMap which eventually increase time for adding an object into Hash Map.

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